Misunderstanding M591?

I'm setting up the laser sensor, seeing how it runs on some general prints.

My understanding is that the Raa:bb arguments correspond to the measured min/max in the output from M591 D0, and that it should be set with a bit of a buffer either side of what's measured - is that correct?

I've been getting mins between -5% to -1%, but when I set negative numbers, M591 D0 seems to move the whole range to start at 0. E.g. R-10:50 would be reported as 0% to 60%, and then pauses when it does hit a minus.

This is all ignoring what a -% means. Does it mean the sensor thinks the filament is going in the opposite direction to what the duet has commanded?

As I had some parts I had to print, I'm still using the filament I need for that -Spannerhands Yellow translucent PETG - so maybe not ideal! Once these parts are done, I can try some different PLAs.

It's not normal to get a negative minimum figure. I suspect on of the following:

The filament monitor needs to be close to the extruder driver, with only a short piece of PTFE tubing (preferably Capricorn) between them. is that how you have it?

If you printed the filament monitor housing yourself, did you print it in black plastic, and are the dimensions accurate - in particular, does it constrain the filament to pass over the centre of the sensing element?

It's not normal to get a negative minimum figure. I suspect on of the following:

The filament monitor needs to be close to the extruder driver, with only a short piece of PTFE tubing (preferably Capricorn) between them. is that how you have it?

Yep, it's within 50mm.

If you printed the filament monitor housing yourself, did you print it in black plastic, and are the dimensions accurate - in particular, does it constrain the filament to pass over the centre of the sensing element?

I believe it's all accurate - but, I used the same filament that I'm currently using (Yellow PETG) - so that might be the issue. Once I have all the parts I'm printing now done, I'll re-print the housing in something darker and solid.